The Witcher and the Empress endings both have their pros and cons. I had the Empress ending and for some reason CDPR had to make it extremely sad. She'll obviously make a bigger impact as an empress but I really doubt she cut out for it. I felt obliged to at least offer her to hear out her father and Charles Dance's voice certainly helped him a lot.
Reading the books currently and I've already heard people say that Emhyr will turn out to be a proper dick. So I'm pretty sure I'll discourage her from meeting the emperor in my next play-through.
As for Triss vs Yenn.. It's gotta be Yenn. Don't get me wrong, I love redheads but Yennefer is just such a fantastically interesting character. The first two books feature some really heavy and neat scenes that tell a lot about the relationship between Geralt and Yennefer.
Currently halfway through the third book in which Triss is really introduced. Triss is a sweetheart but there's a very dark side to her too, which the games don't really show. Twisted obsession. She's also had more of a big sister role to Cirilla.
As for politics.. I don't really know. If Ciri sits on the throne and succeeds, she could usher in a great new era. If she can keep her head cool with all the people and interests nagging at her.. Eilhart, The lodge, Nobles, Unrest in the newly conquered regions, racial tensions etc etc. Who knows, she's the prodigal child. And hey, perhaps she'd take up her father's promise with Letho and rebuild the Viper school. Pimp up Kaer Morhen a little. Have Papa VeseGeralt and VeseLetho train new witchers for a few centuries. A great human empire under Cirilla's reign could even stand against the Aen Elle. Imagine the empire's legion marching into the Empress' portals, showing those snobby elves who's boss. A war between worlds.
If Ciri's becomes a Witcher I don't really care much for the politics. As long as Radovid is dead. Stalin Dijkstra will keep the north fighting until the bitter end, Emhyr will die and perhaps the North could even push back. A united Nilfgaard under Emhyr's rule would prevent further bloodshed (apart from his former enemies, which he executes). But hes supposed to be kind of a prick... But he's also Tywin Lannister which makes it kind of cool I suppose.
If we're talking sequels/canon, I wouldn't want to play Witcher Ciri the same way we played Witcher Geralt. Geralt, although a great warrior, is still a very flawed person. He has (had) many strugles to overcome and his powers have limits. Ciri on the other hand is like a deity incarnate. I'm reading the books and I'm seeing some flaws, sure... But shes still the prodigal child; The Dovahkiin, The Inquisitor etc. If she learns to control those powers she'd be essentially omnipotent. Ciri OP.
If the Witcher had a sequel I'd like Ciri to be a succesful empress. Witchers are somewhat redeemed and no longer scorned as they were. Geralt and Letho are the new Vesemirs and are training new witchers. The racial tensions somewhat eased. Magicians having some sort of reasonable position in society.. For a plot though, idk.. Some kind of great otherworldly threat maybe. Some shit going on in Zerrikania perhaps. The empire's collapse a la game of thrones.
Plenty of room for prequels too ofc.